Power circuits are employed in a variety of applications such as AC/DC conversion circuits, DC/DC conversion circuits, etc., in which developed taking conversion efficiency, power efficiency, etc., must be taken into consideration. Switching regulator type power circuits are mainly used for large power consumption devices, while series path type power circuits are used for other devices requiring a precision output voltage. Switching speeds and ripple factors, etc. must be considered when using switching regulator type power circuits. Losses in circuit elements, transmission efficiencies, etc., must be considered when using series path power circuits. The effective power of an AC circuit is determined by the phase between voltage and current. The performance relating the phase is generally expressed as a power factor.
FIG. 1 shows a conventional power circuit.
In FIG. 1, an AC power source PS1 is connected to a full-wave rectifier circuit DB1. The full-wave rectifier circuit DB1 supplies its rectified output to a switching regulator, i.e., the load circuit of the full-wave rectifier circuit DB1 after smoothing by capacitor C5. The switching regulator includes a transformer T1 and a switching transistor Q1. A starting circuit comprised of a half-wave rectifier diode D1 and a capacitor C2 is also connected to the switching regulator from the return line in the single phase AC loop of the full-wave rectifier circuit DB1 via a resistor R2.
A DC power output VCC is supplied to a voltage regulating circuit IC1 from a half-wave rectifier circuit D2 coupled to a tertiary winding L4 of the transformer T1 after smoothed by capacitor C3. The output of the voltage regulating circuit IC1 is supplied to the base of the transistor Q1.
The collector of the transistor Q1 is connected to one end of the primary winding LP of the transformer T1 and the other end of the primary winding LP is connected to the smoothing capacitor C5 and the rectifier circuit DB1.
An AC voltage induced across secondary winding L2 of the transformer T1 is rectified through a diode D3 and then the rectified voltage from the diode D3 is output from the output terminal 1 of the power circuit as a stabilized output after being smoothed by capacitor C4 coupled between the cathode of the diode D3 and the reference potential line.
An error amplifier IC2, detecting fluctuating components of the output voltage, is connected to the cathode of the diode D3. The output of the error amplifier IC2 is connected to the cathode of a control diode A1 which comprises a part of a photo-coupler and then the anode of the control diode A1 is connected to the output terminal 1 via a resistor R5. The emitter of a transistor Q2 constituting the other part of the photo-coupler is coupled to the control terminal of the voltage regulating circuit IC1 while the collector is connected to DC power VCC output of the half-wave rectifier circuit D2 through a resistor R4.
Next, the operation of the conventional power circuit, as shown in FIG. 1, will be explained using the operating power waveform, as shown in FIGS. 3(d) through 3(e).
Due to smoothing capacitor C5, during the period .tau., as shown in FIG. 3(a) when an input AC voltage VAC is higher than rectified output voltage E.sub.1, in other words, the period when the rectified output voltage Ei drops below the AC input voltage VAC by being supplied to the adding circuit, a pulsating AC current, as shown in FIG. 3(b) flows through the rectifier diode DB1 to turn ON the diode DB1. Further, FIG. 3(c) shows the collector current of the transistor Q1.
Generally, the smoothing capacitor C5 cannot be too small when ripples contained in the output voltage EB of the switching regulator are taken into account. In this case, the power ON duration .tau. of the rectifier diode is extremely short.
According to the actual measurement, the power ON duration .tau. is approximately 2 to 2.5 mS when C1 is 470 .mu.F and the load power is 80 W.
Therefore, the case of the circuit shown in FIG. 1, the power factor is as low as 0.6 (60%) and a harmonic waves current contained in the pulsating current in the AC power source is also large. In order to increase the power factor and to reduce the harmonic waves current, it is necessary to extend the ON duration .tau. of a diode. In general, when current i (t) is expressed using a Fourier series, the following Equation 1 will be obtained: ##EQU1##
Here, if i(t) is a unit step function, as shown in FIG. 4(a), a DC component a.sub.0 and AC components a.sub.n, b.sub.n will be expressed by the following Equation 2. ##EQU2##
If the r.m.s. value of the fundamental wave current where n=1 is i.sub.1, the r.m.s. value of the harmonic waves current is i.sub.n, and the r.m.s. value of i(t) is i.sub.rms, relationship among i.sub.1, i.sub.n and i.sub.rms is given by the following Equation 3. ##EQU3##
If the power ON duration .tau..sub.2 of the diode in the above Equation 3 is extended, the fundamental wave current i.sub.1 increases and a power factor also increases. On the other hand, the harmonic wave current i.sub.n decreases.
The power circuit, as shown in FIG. 2, has been devised improve this power factor. The power circuit of FIG. 2 is an example where a MOSFET switching transistor Q3 was used along with other parts are identical to the circuit of FIG. 1 except that the smoothing capacitor C5 was not used after rectification. The power circuit of FIG. 2 is also an example where the power efficiency was improved by the operation of a voltage driving type device by making the most of operating characteristics such as the switching speed, etc., and by utilizing a rectified pulsating voltage.
Smoothing capacitor C5 is not provided in FIG. 2 after the AC power was rectified and a switching regulator is operated directly by the AC voltage.
In this case, since the switching transistor Q3 operates over the whole period T, correspondingly the ON duration of the rectifier diode extends to T/2 and the value of the power factor obtained is above 0.9.
While there is a power factor improvement, there are other drawbacks that will be explained below.
The drain-to-source current i.sub.DS flowing through a. MOSFET switching transistor Q3 in the operating state is shown in FIG. 3(d). Its envelope has a sine-wave shape, and thus the drain-to-source current i.sub.DS becomes small in a time period where AC voltage is low, while it becomes large when the AC voltage reaches around a peak value.
The collector current i.sub.CP of the transistor Q1 in FIG. 1 is smoothed, as shown in FIG. 3(c), by the smoothing capacitor. Therefore, when this collector current is compared with the drain-to-source current i.sub.DS of the transistor Q3 at a same load (the same mean current) condition, the drain-to-source current i.sub.DS of the transistor Q3 becomes two or more times the collector current i.sub.CP. Because of this, the rating of the transistor Q3 (MOSFET) becomes large and it becomes necessary to make the large transistor in connection with the saturation of the switching transformer core and thus manufacturing will increase.
The circuit shown in FIG. 2 is an example where a current driving type bipolar transistor was changed to a voltage driving type bipolar transistor and the characteristics of a power source using the MOSFET type switching transistor Q3 are improved in view of characteristics such as the switching speed, the input impedance, etc. However, as far as cost is concerned, there is cost increase as over-ratings and over-specifications of performance are demanded and one is forced to use larger parts to accomodate the maximum operating range when compared with the bipolar transistor.
Since the second rectified output voltage of the switching regulator has a sine-wave enveloped ripple voltage, as shown in FIG. 3(e), it becomes necessary to provide a switching regulator using a series path type regulator or a choke coil at the secondary side.
The operation holding time during the momentary stoppage of AC power is too short because no smoothing capacitor is provided. Therefore, there is the problem that the output voltage EB drops largely due to the fluctuation in the input line or a momentary fluctuation of external noise and distortions will appear on the screen when the invention is applied to a TV set. The product performance is thus deteriorated.
Since the current flowing through the switching transistor has the sine-wave envelope, as described in reference to the first drawback, if a current driving type bipolar transistor is used as a switching element, the base current (the driving current) also must be modulated to the sine-wave envelope.
However, there was a problem in that it was actually difficult to use a current driving type transistor because of a difference between ON and OFF durations or restriction of switching speed of a current driving type PN junction transistor which had to use a voltage driving type MOSFET.
As described above, a conventional circuit has the drawbacks of a low power factor and the power efficiency is worse. A voltage driving type FET is used as a switching element, its power efficiency can be improved by characteristics such as the switching speed, and the input impedance, etc. However, there was a problem that in view of the cost, it is expensive and furthermore, the number of additional circuits as well as component parts will increase and the circuit will become larger.